1. Field of the Invention
The present invention relates to fasteners employed to fasten a covering material to an underlying substrate. More particularly, the invention relates to a plastic/metal composite stress plate with a fastener for fastening a membrane, such as a roof membrane or roofing insulation, to a roof deck, a wall, concrete, stone, plaster, or wood.
2. Reported Development
Fasteners are conventionally employed in the building industry for fastening or clamping a flexible membrane, such as an insulation membrane to a substrate, such as a roof deck. The fasteners typically comprise a large head portion and a shank portion. In use, the shank portion is driven through the membrane into the underlying substrate to anchor the fastener thereinto, while the head portion holds the membrane against the substrate and prevent removal thereof by wind lift. The undersurface of the head portion is typically provided with gripping means so that the membrane is prevented from moving or sliding under the fastener. The gripping means are designed not to penetrate the membrane in order to prevent atmospheric moisture from entering into the substrate through the holes which tend to be made by the gripping means. It is also important that the gripping means are spread/distributed in the undersurface of the head portion of the stress plate in order prevent tearing of the membrane. Conventional fasteners are illustrated by the following references.
U.S. Pat. No. 4,787,188 discloses a stress plate for securing a roof membrane to a roof deck. The stress plate is circular having a top surface and a bottom surface with a central circular opening for receiving a screw for fastening the stress plate over a roof membrane and to the roof deck. The stress plate is equipped with four gripping prongs of triangular shape which are circumferentially spaced from each other by 90°.
In use a first membrane is applied to a roof deck surface, then the membrane is secured to the roof deck surface with the stress plate and the screw. A top sheet or membrane is lapped over the first membrane to cover the stress plate and welded to the first membrane. The four gripping prongs in the stress plate grip the first sheet and hold the same on top of the roof deck without tearing.
U.S. Pat. No. 5,049,018 discloses a fastener for gripping a substrate material. The fastener is of a unitary piece comprising a head portion, a shaft portion, and a hook portion at the end of the shaft portion, wherein the hook portion has an outwardly and upwardly extending resilient end portion. The end portion has an end surface which provides gripping contact with a wall of a hole in a substrate into which the fastener is inserted.
U.S. Pat. No. 5,163,798 relates to a fastener assembly which is employed to secure plies or membranes of roofing, felt and paper to prevent the materials from being blown off the base roofing material before the base material is sufficiently hardened.
The assembly comprises a fastener and a retainer plate. The assembly includes a fastener plate which defines a substantially rectangular opening. The fastener includes a head and a pair of legs which are integrally hingably connected to the head. The legs have a contoured distal portion and an angular side configuration so that at least one of the legs is forced apart as the fastener is driven into the base material.
We have observed that under windy conditions the prior art fasteners need improvement in securely holding a flexible membrane on a substrate without the gripping means penetrating the flexible membrane, and without tearing the flexible membrane.
Accordingly, an object of the present invention is to provide a new and improved stress plate with a fastener to allow attachment of one or more flexible membranes to an underlying substrate without tearing the flexible membrane or allowing it to slip out from under the stress plate.
In another aspect, the present invention relates to a method for securing thermoplastic roof membranes to a roof deck using a stress plate and fastener and fusing overlapping portions of two roof membranes to provide a waterproof covering over a roof deck.
Asphalt roof membranes to prevent moisture from entering into an underlying roof deck are being replaced by thermoplastic sheet materials which offer a superior, longer-lasting roof at a lower cost. In the process of installing thermoplastic sheet materials over a roof deck, the typical steps are as follows. A first sheet is laid adjacent to the lower edge of the roof and running parallel thereto. Fastening means, such as batten bar or a line of stress plates are positioned neat the upper edge of the first sheet. Fasteners are inserted through the batten bar or stress plates and into the roof deck to securely hold the first sheet to the roof deck. Then a second sheet is laid over the roof deck in a marginally overlapping relationship with the first sheet. The second sheet also overlaps the batten bar or the line of stress plates. The overlapping edge area of the second sheet overlaps the area of the first sheet on both sides of the batten bar or line of stress plates. A weld is then applied between the first sheet and the second sheet resulting in the fusion of the two sheet on both sides of the batten bar or the line of stress plates. The weld is applied by the use of a welding machine or tool which softens the thermoplastic sheets and, after cooling, solidifies and forms a continuous sheet. The steps are repeated until the roof deck is completely covered by the thermoplastic sheets.
It has been observed that stress plates having a high profile create bumps in the weld area. It has also been observed that stress plates made of metals do not adhere to the second or overlapping thermoplastic sheet thereby creating bubbles in the weld. Accordingly, it is another object of the present invention to provide a low profile stress plate which substantially remains in the plane of the thermoplastic sheets and which is coated with a thermoplastic material so that the second or overlapping sheet is also welded to the stress plate.
U.S. Pat. No. 6,640,511 discloses an anchor plate with a fastener. The plate has an elevated bonding platform and a countersink. The bonding platform is coated with a heat activable adhesive. The other top surfaces are not coated with the adhesive.
The reference uses an electromagnetic induction heater for attaching a thermoplastic membrane to the anchor plate and the underlying roof structure.
Attaching thermoplastic membranes to a roof surface by using electromagnetic induction heat is cumbersome.
These and other aspects will be addressed as the description of the invention proceeds.